The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads is a commercially significant application of polymer film, including generically, polyethylene. Overwrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Another wrapping method finding acceptance in industry today is that of hand wrapping. In this method, the film is again arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The roll of film so used may be installed on a hand-held wrapping tool for ease of use by the operator.
Historically, higher performance stretch films have been prepared with m-LLDPE, most often with the m-LLDPE located in an interior layer. Such films have shown markedly improved puncture and impact resistance as well as improved film clarity relative to counterparts made with more traditional Ziegler-Natta LLDPE""s. Stretch films employing higher amounts (up to 100 wt %) of m-LLDPE either as a discrete layer or layers, or as a blend component in a discrete layer or layers of a multilayer stretch film, propagate defects more easily leading to web breakage. This defect propagation has precluded the development of film structures containing higher concentrations of m-LLDPE to maximize toughness. The results of this work show that stretch films with significantly improved defect propagation resistance relative to the following film types can be made: 1) A five-layer (A/B/C/B/A) stretch film formulation common in the stretch film industry where A=C=Ziegler-Natta ethylene-hexene LLDPE and B=m-LLDPE; 2) A stretch film comprised of 100 wt % ethylene-hexene or 100 wt % ethylene-octene Ziegler-Natta copolymer.
Film Testing Methods: MD and TD refer to the machine direction and transverse direction, respectively, as they relate to cast film production. Film Gauge (Exxon PLFL-238.001), Laboratory Puncture Force (Exxon PLFL-201.01), Elmendorf Tear (ASTM D1922-94); Cling (Exxon PLFL-201.02 based on ASTM D5458-95), Melt Index (ASTM D1238-94), Density (ASTM D1505-96, compression molding of samples by ASTM D1928-96), FDA Hexane Extractables (21 CFR 177.1520(d)(3)(ii)). The Highlight Ultimate Stretch Test and the Highlight Puncture Test were each conducted in accord with Highlight Industries, Inc. Film Development Test System Operations Manual (Copyright, 1996).
In this work, a defect was introduced by way of a Highlight Stretch Tester Puncture Test (at progressively higher levels of stretch). Destruction of the web was designated by us as Failure Mode (FM) 3, the type of film failure deemed most undesirable because web failure requires more operator attention and effort in stretch wrap applications as those skilled in the art of stretch film know. FM2 referred to a film puncture, but no defect propagation. While still undesirable, the web was not destroyed, merely damaged by a hole in this mode of failure. In FM1 the probe did not puncture the film and would be the most desirable.
This research disclosure shows how 5-layer film structures can be prepared with higher levels of m-LLDPE, particularly in the skin and core film layers, that do not propagate a defect (FM3) as discussed above, thereby maintaining the integrity of the web during use. Other performance benefits were noted from certain film structures prepared in this work such as an optimal balance of stiffness and extensibility, a minimization in cling force reduction upon stretching, and a minimization in unwind force. Some film samples (such as samples 004 and 007) were highly extensible, and required higher levels of force to stretch the film in the Highlight Ultimate Stretch Test. In some applications, this combination of stiffness and extensibility is preferred. Also noted in this work was that the reduction in cling force with film stretching generally experienced by those skilled in stretch film was minimized for some film samples. By way of example, cling force at 200% stretch was higher for samples 004-005 than for samples 014-015. Most noteworthy was the higher cling force at 200% stretch of samples 001-011 relative to sample 013, prepared from ethylene-octene LLDPE. Finally, while the relationship between unwanted increases in unwind force and higher extractables concentrations are well known to those skilled in the art of stretch film, some of the film samples prepared herein exhibited dramatic increases in unwind force and unwind noise unrelated to extractables. The root cause of the unwind force increase was strain-induced crystallization. Steps to mitigate strain-induced crystallization and thus avoid unwanted increases in unwind force will be addressed.